The present invention relates to a received signal determination method, a positioning device, and an electronic instrument.
The global positioning system (GPS) is widely known as a satellite positioning system. The GPS is utilized for a positioning device provided in a portable telephone, a car navigation system, and the like. A GPS positioning device acquires and tracks a GPS satellite signal transmitted from a GPS satellite, decodes a navigation message included in the GPS satellite signal, and calculates a pseudo-range based on GPS satellite orbit information and time information included in the decoded navigation message to calculate the current position of the GPS positioning device.
The GPS positioning device acquires the GPS satellite signal (GPS satellite) by performing correlation calculations on the received signal and a code replica. The code replica is a signal that is pseudo-generated by the GPS positioning device and simulates a pseudo random noise (PRN) code included in the acquisition target GPS satellite signal. If the acquisition target GPS satellite signal is correct, the PRN code included in the GPS satellite signal coincides with the code replica. If the acquisition target GPS satellite signal is incorrect, the PRN code included in the GPS satellite signal does not coincide with the code replica. Therefore, the GPS positioning device can determine whether or not the GPS satellite signal has been acquired successfully based on the correlation value calculated by correlation calculations.
In general, a given threshold value is set in advance, and the GPS positioning device determines that the GPS satellite signal has been acquired successfully when the correlation value calculated by correlation calculations has exceeded the threshold value. However, even if the PRN code included in the GPS satellite signal does not coincide with the code replica, the correlation value may exceed the threshold value in an environment in which a strong signal can be received, for example. Therefore, the GPS positioning device may incorrectly determine that the GPS satellite signal has been acquired successfully. This phenomenon is referred to as cross-correlation, and may decrease the positioning accuracy to a large extent.
As technology that solves this problem, JP-A-2003-98244 discloses technology that detects cross-correlation using the difference in Doppler frequency between signals received from GPS satellites.